There is a need for treads for tires that have excellent grip performance on both wet and dry roads. It has been known that the grip performance, especially wet grip performance, of rubber compositions for treads can be improved by incorporation of α-methylstyrene-based resins (see, for example, Patent Literature 1). Other known methods include the incorporation of terpene-based resins such as polyterpene, terpene phenol resins, or terpene aromatic resins.
In order to improve dry grip performance, it is necessary to improve tan δ at 20° C. Unfortunately, this at the same time leads to an increase in tan δ at 30 to 70° C., which is an indicator of rolling resistance, and is thus not desirable. For example, known methods include the use of Koresin, coumarone-indene resins, styrene acrylic resins, or the like; however, these methods at the same time lead to greatly increased rolling resistance and are thus difficult to use in applications where fuel economy is important.
In general, resins used to improve grip performance (grip resins) function to increase the viscoelastic properties, hysteresis loss, and especially tan δ of rubber. In addition, such resins also function to increase the adhesion between the tread and the road surface as they bloom to the tread surface during running, thereby forming an adhesive bloom layer together with process oils, polymer decomposition products, low molecular weight organic materials, and other components. It is considered that the above effects together contribute to the improvement of grip performance.